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Water Efficiency at Facebook's Prineville Data Center

For Facebook, good data center design is all about efficiency — how efficiently we use energy, materials, and water, and how they tie together to bring about cost efficiency. We've previously shared information on energy and materials efficiency, and today we're releasing our first water usage effectiveness (WUE) measurements and information on how we've achieved what we think is a strong level of efficiency in water use for cooling in the first building at our Prineville, Ore., data center (which we'll call Prineville 1 here).

We're sharing this information as part of our commitment to the Open Compute Project; we believe that open sharing and collaboration surrounding these kinds of best practices is crucial to ensuring that, as an industry, we're innovating rapidly and minimizing our environmental impact.

After a year of operation, which required our retrofitting the building management system and adding water metering, we've found Prineville 1's Q2 2012 WUE to be 0.22 L/kWh. By definition, WUE is an annualized calculation; however, we will report results on a quarterly basis, and those numbers will eventually become a 12-month trailing metric. WUE measures water used for cooling a data center only; it doesn't measure water used for plumbing or offices, though Facebook minimizes water usage in these areas as well by using reclaimed water and waterless urinals in the bathrooms.

We think that 0.22 L/kWh is a great result, but it should be noted that the WUE concept is fairly new and, to our knowledge, no one else has publicly reported WUE yet. We hope that other companies will soon start measuring and reporting WUE so we can begin setting benchmarks for the metric and working together to find new ways to improve.

How Facebook Minimizes Water Usage for Cooling

Prineville 1's mechanical system comprises a built-up penthouse that utilizes 100% outside air economization with a direct evaporative cooling and humidification (ECH) misting system. This design allows us to achieve a strong WUE. Details and specifications for this system can be found at www.opencompute.org/specs. This blog post provides insight into the water efficiency of the Open Compute Project (OCP) mechanical system deployed in Prineville.

Mixing outside air and return air. (Photo by Alan Brandt.)

Typical data center mechanical systems usually do not use outside air economization, instead recirculating up to 100% of the air used to cool the server room with a central chilled water plant and cooling towers, which consumes much more energy and water. It's like using a window-mounted air conditioner to cool a room instead of putting a fan in a window when the outside temperatures are cooler than the temperature in the room.

Mixing outside air and return air. (Photo by Alan Brandt.)

Furthermore, the typical data center mechanical system consumes water via cooling towers, which process waste heat to the atmosphere via evaporation as a large fan blows air over media moistened with water. In addition to this evaporation, cooling towers require blow down, which is the dumping of the cooling tower water to sewage. Blow down is necessary for a cooling tower due to the water accumulating dissolved minerals as the water evaporates.

The OCP mechanical system has no chillers or cooling towers. Instead, it uses outside air as a first stage of cooling, a process that is also referred to as outside air economization. Economization is utilized the entire year in Prineville 1, as the OCP system is designed to be single pass, where outside air enters the data center, warms up as it passes over the servers, and then is either recirculated or exhausted back outside. This system greatly minimizes the need for blow down, and it also doesn't require the use of chemical treatments to combat biological intrusions like algae.

The second stage of cooling occurs via the direct ECH misting system. The ECH system drops the temperature of the air significantly via phase change of liquid water to water vapor in the direct path of supply air to the data hall. Evaporative cooling technology has been used for thousands of years for built environments. Perspiration is also a simple example of evaporative cooling.

The ECH system is composed of the following equipment:

Water storage tanks

Booster pumps

Carbon filters

Water softeners

Reverse osmosis (RO) water purification skids

RO water storage tanks

Distribution pumps

Misting system pump skids

Mist eliminator

Water polishing system

The booster pumps take water from the outdoor water storage tanks and pump the water through carbon filters for initial filtration. Water softeners then precipitate calcium, magnesium, and other minerals. Reverse osmosis (RO) skids then further purify the water of total dissolved solids. Our measurements indicate that 75% of the water brought into the data center is used for cooling (3 out of 4 parts of RO water are used for product), while the remaining 25% is blown down. The main purpose of the RO system is to purify the water to minimize the potential to clog the misting nozzles, as the ECH system includes thousands of high pressure misting nozzles with 150 micron diameter orifices, and using untreated water can create a maintenance issue because it can clog the nozzles.

Prineville 1's misting system. (Photo by Alan Brandt.)

Following purification, the RO product water is pumped into RO water storage tanks. Distribution pumps then pump the RO water at 40 PSI up to misting system pump skids that pump the water into stainless steel piping at 1000 PSI upon demand to meet temperature or humidity setpoints. The misting pump skids are equipped with variable frequency drives (VFDs) that can provide very fine-grained control of the misting system for accurate temperature and humidity controls.

The stainless steel piping is routed to the misting nozzles, which are set up in an array, where the nozzles atomize the water, maximizing the misting water surface area – which in turn maximizes the evaporation rate. Approximately 85% of the misted water evaporates into the supply air stream. The remaining 15% is recaptured in a mist eliminator that is downstream of the misting nozzles. The recaptured RO water is put through a micron filter and UV lamp and then brought back to the RO water storage tanks, which is another method of water conservation designed within the system. The air is then delivered into the data center via fans that push the air down dry wall supply airshafts.

The end result is a mechanical system that is more water efficient than a typical chilled water/cooling tower data center mechanical system. The design of Prineville 1 inherently uses less water as we require less hours of "cooling" since the OCP mechanical system uses free cooling via economization and can operate at higher temperatures. Higher operating temperatures require less energy to cool the air Additionally, the ECH system consumes less water – it's a more-direct method of heat transfer to the supply air into the data hall, as it's in the supply air stream, rather than a cooling tower that requires blow down.

Reporting WUE in the Future

Facebook will continue to release WUE metrics on a quarterly basis for all our data centers. The WUE for the second building at our Prineville data center should be available next year, when that building is online. Facebook is also working on WUE metrics for our Forest City, North Carolina, data center, and will report once we have enough data.

If you're interested in learning more about how we designed our Open Compute data centers, download the electrical and mechanical specifications from the Open Compute website. To join the conversation, subscribe to the data center design mailing list If you're interested in helping the industry tackle these efficiency issues and more, get involved with the Open Compute Project.